Organic Chemistry: Concepts and Applications - Headley Allan D. 2020

Alkanes, Cycloalkanes, and Alkenes: Isomers, Conformations, and Stabilities
4.8 Stability of Alkynes

There are no geometric isomers of alkynes, but structural isomers are possible in terms of the location of the triple bond. The triple bond can be internal or terminal, and Reactions (4-5) and (4-6) give the hydrogenation of a terminal and internal alkyne.

(4-5)∆−−

(4-6)∆−−

The energy profile for Reactions (4-5) and (4-6) is shown in Figure 4.30.

∆−∆−

Figure 4.30 Reaction profile for the catalytic hydrogenation of 1-hexyne and 3-hexyne.

Since the least amount of heat is liberated from 1-hexyne, compared to 3-hexyne, it can be concluded that 3-hexyne is more stable than 1-hexyne, and a general conclusion can be drawn from this observation that alkynes that have internal triple bonds are more stable than alkynes with terminal triple bonds.

Problem 4.14

Of the following pair of isomers, determine which is more stable.

1. (E)-3-hexene and (Z)-3-hexene

2. 1-pentene and 2-pentene

3. 2,3-dimethylcyclohexene and 1,2-dimethylcyclohexene